When a vehicle is involved in an accident such as a rollover or collision, it is valuable to record the rotation angle of the vehicle before and after the accident. The recorded rotation angles make it possible to analyze the behavior of the vehicle and investigate the cause of the accident. In particular, the roll angle around the X-axis during the rollover illustrated in FIG. 1 is may be valuable. For example, Japanese Patent Publication No. 2003-72600 (Patent document 1) discloses a technique for recording the angular velocity before and after vehicle collision.
While an angular velocity sensor continually measures the angular velocity of a vehicle, the data required for analyzing vehicle behavior include the rotation angle of the vehicle just before and after the accident. Thus, there is no need to record vast data over a long period until an accident occurs. A ring buffer is a means for storing only a latest part of the vast data (disclosed for example in Japanese Patent Publication No. 2007-45221 (Patent Document 2)). Conceptually, a ring buffer is a memory arranged in a ring shape around which a write pointer moves circularly to replace the oldest data with new data, thereby continually storing a latest certain amount of data.
Use of a memory storage mechanism such as a ring buffer makes it possible to quickly store a certain amount of data. However, because the memory is volatile, such data needs to be moved to a non-volatile recording device when an accident occurs in order to preserve the data. Japanese Patent Publication No. 2006-151006 (Patent Document 3) discloses a technique for recording various data in a recording device when a value detected by an acceleration sensor of the vehicle exceeds a predetermined threshold.
However, with regard to Patent Document 1, this patent does not disclose a specific method for recording the rotation angle of a vehicle. Given that angular velocity is to be stored, the angular velocity needs to be measured and stored at an extremely short sampling interval of about 0.02 seconds, for example. Such a short sampling interval is required to sufficiently obtain the accuracy of the rotation angle calculated after an accident. In this case, however, several hundreds of angular velocity need to be stored in a buffer over a period of several seconds before and after an accident. Hence, an increase of buffer capacity may be required.
One possible solution may be to integrate the angular velocity detected after the angular velocity sensor begins measuring the angular velocity in order to record the total rotation angle of the vehicle every predetermined time point. For example, Japanese Patent Publication No. 2002-267500 (Patent Document 4) discloses a technique for integrating waveform detected by a sensor. However, when the angular velocity sensor has even the slightest offset (initial error) or noise, integration is continually performed with that offset or noise. As a result, the total rotation angle readily exceeds the storable range of the buffer, resulting in saturation and failure to store the necessary data.
The angular velocity sensor measures angular velocity by sensing an extremely low voltage, and is thus readily susceptible to offset and noise. While solutions such as ignoring the noise (establishing a dead zone) or applying a bias to converge measured values to zero are possible, these solutions are generally not accurate and require much time for processing.
Further, the angular velocity sensor has a measurement range and cannot output a value that exceeds the range. Thus, it is wasteful to prepare a recordable area in a recording device for recording rotation angles that will never be attained. Also in view of maintaining the resolution of recorded values, such a needless recordable area should be minimized.